How to configure blender so that it uses only 50% of total CPU when rendering?
While it's often more efficient to leave this to the Operating System, to answer your question specifically with a method that works, look under the Performance Panel in the Render Settings. It should be set to use the max amount of CPU threads by default, click Fixed and set it to half of however many threads your CPU has. It might look a bit different for BI and Cycles but the Threads part is what you want.

Obviously, you need a CPU with multiple cores for this to work. If you are on a single core machine, altering the value under Fixed will have no effect and your machine will also not use more threads that the value in Auto-detect.